The present invention relates to a method of driving a semiconductor laser, and more particularly to a driving method which suppresses laser noise in the case of employing a semiconductor laser as an information processing light source.
Semiconductor lasers are being put into practical use as pickup light sources for an optical video disc, an optical digital audio disc, etc. Since, however, the coherency of the semiconductor laser is too good, there is the problem that the laser output fluctuates due to the feedback of reflected light from an external optical system. Laser noise attributed to the feedback of the reflected light develops conspicuously in the semiconductor laser which oscillates in the single mode under direct-current drive. Further, the single mode laser has the problem that, even when the feedback of the reflected light is not involved, two or more modes oscillate competitively within a certain temperature range, to increase laser noise.
As a method for suppressing the laser noise of the single mode laser, a method wherein the laser is driven by superposing a high frequency current and is brought into multiple mode oscillation has been proposed in Japanese Patent Application No. 55-113515 filed Aug. 20, 1980 (changed from Japanese Utility Model Registration Application No. 54-99374 (filed July 20, 1979), and corresponding to Japanese Patent Application Laying-open No. 56-37834 (laid open Apr. 11, 1981), West-German Application No. P 3027318.8 (filed July 18, 1980) and U.S. Application Ser. No. 169794 (filed July 17, 1980), now abandoned and and refiled as Ser. No. 472,220 on Mar. 4, 1983, which issued as U.S. Pat. No. 4,480,325 on Oct. 30, 1984) which are hereby incorporated by reference regarding the state of the art in the field. In the multiple mode oscillation state, the coherency is lowered, and hence, the laser noise developing due to the feedback of the reflected light weakens. The laser noise attendant upon the temperature change, however, cannot always be suppressed merely by the high frequency superposition.